A vehicle's engine may include various devices to improve fuel economy and reduce emissions. One such device includes variable valve actuation, such as variable cam timing and/or variable valve timing. Various approaches have been used to diagnose degradation of such devices, where such degradation may result in increased emissions.
For example, in the example of Variable Cam Timing (VCT), cam position over advanced from scheduled VCT position may result in reduced residual gas, which may increase NOx emissions. Likewise, cam position over retarded from scheduled VCT position (especially at lower speeds) may result in excess residual thereby diluting combustion, which may result in slow or partial burn and thus increase HC and CO emissions.
One approach that attempts to diagnose position errors is described in EP 1 363 007. In this example, diagnostics for a feedback position control system are based on a difference or ratio between and integral value of the target position and an integral value of the actual position.
The inventors herein have recognized a disadvantage with such an approach. In particular, the inventors herein have recognized that the effect of valve timing errors on emissions may be affected by more than position errors. In other words, while errors from over advanced or over retarded cam position may affect emissions, variation in cam position between cylinders (such as between banks of cylinders) may also affect emissions. Further, these parameters can interact to have compound effects on emissions. As one example, if two banks in a V-engine have different cam positions, this may result in sub-optimal spark advance and/or open loop fueling, each of which may increase emissions.